The goal of this project is to characterize monocyte-macrophage responses to extracellular microorganisms and to relate these responses to protective immunity. Using the extracellular protozoan Giardia lamblia as a prototype extracellular organism, we will probe the mechanisms of monocyte chemoattration and cytotoxicity for G. lamblia and the pathways of monocyte and lymphocyte immunoregulation of host responses to this organism. Results from these studies will increase our understanding of how monocyte participate in protective immunity against extracellular microorganisms.